The mentioned introduction of the reference air can bring moisture into the pressure chamber, which then condenses in the interior of the sensor when the temperature falls below the dew point. This can degrade the functioning of the sensor. Such is especially a problem, when the air surrounding the sensor has a higher temperature than the medium whose pressure is being measured.
Hegner et al. disclose in European Patent Application EP 0 974 825 A3 a relative pressure sensor, which has a reference air path with a moisture filter. The moisture filter is arranged in the front area of the relative pressure sensor, near the measuring membrane, thus near the medium, so that the temperature of the moisture filter is similar to the temperature of the media. This arrangement assures that moisture in the reference air can condense out before it even gets into the reference air path, so that there is hardly any possibility of a falling below the dew point in the pressure chamber. The described arrangement is, however, comparatively costly.
Other relative pressure sensors have a capillary tube communicating with the platform-side opening of the pressure chamber. The capillary tube serves as a reference air path, while opposing encroaching moisture with a certain diffusion resistance. The capillary tube is frequently a small metal tube, which, for example, is secured to the platform by glazing. This manner of assembly is accompanied, likewise, by an increased manufacturing cost. Additionally, the entrance opening of the capillary tube is largely thermally de-coupled from the pressure measuring cell, so that, in the case of higher temperatures at the entrance opening, air with a high water content can get into the capillary tube, which leads to falling below the dew point in the colder pressure chamber. The condensation in the measurement cell is significantly delayed and even decreased by the described arrangement, but it cannot, however, be prevented by this technique.